1. Field of the Invention
This invention relates to smoke and obscurant pyrotechnic compositions. More specifically, the present invention relates to low toxicity smoke-producing compositions that have an obscuration index at least as great as hygroscopic chloride (HC) smoke.
2. Technology Background
Prior art and experimentation teach that obscuration efficiency is a function of particulate size, refractive index and concentration in the atmosphere. Conventional pyrotechnic obscurant compositions are, therefore, based on materials which generate a dense primary particulate, such as inorganic oxides, or compounds which easily form atmospheric aerosols, such as hydrochloric acid, polyphosphates, or phosphoric acid.
Although various smoke-producing compositions and devices are presently known, many such compositions are toxic. Most smoke-producing compositions incorporate materials which are severely toxic or are irritants when subjected to the heat necessary to produce smoke. Personnel anticipating exposure to such harmful smoke must protect themselves from the smoke. The problem of toxicity and irritation to people is clearly a limitation in several respects. Not only does it increase the potential for injury, but it may dictate the use of additional specialized equipment, such a respiratory protection. This type of equipment is expensive, and in the situations such as training exercises, may detract from the ability to simulate actual conditions.
A related problem is the effect of smoke-producing compositions on equipment and supplies. In addition to being toxic and irritating to people, conventional smoke-producing compositions are corrosive and damaging to both mechanical and electronic equipment. It will be appreciated that this is a major disadvantage under typical operating conditions. Smoke producers are usually employed in field operations which involve the use of precision electronic and mechanical equipment that may be damaged by the corrosive exhaust of such smoke-producing agents. Accordingly, the use of corrosive and damaging chemical compositions is a severe limitation for many known smoke compositions.
For military use, volatile hygroscopic chloride (HC) smokes are important for large scale operations. The most widely used HC type smoke-producing compositions are those resulting in the production of zinc chloride smokes. One example of a military HC smoke composition employs a reaction between hexachloroethane and zinc to produce zinc chloride. However, the reaction products are very toxic and believed to be carcinogenic. This has recently prompting the United States Surgeon General to ban the use of such smokes. Typical HC smokes have an obscuration index of about 200.
Obscuration index is a dimensionless figure of merit for comparing the efficacy of smoke compositions. It compares the transmittance of electromagnetic radiation of a wavelength (or band of wavelengths) at a fixed smoke concentration and pathlength. The following equation, based upon Beer's Law, defines the transmittance of a smoke cloud as a function of mass extinction coefficient, concentration and path length. The transmittance is a function of both wavelength and time in a burning pyrotechnic. EQU T.sub..lambda. (t)=e.sup.-.alpha.CL
where T=transmittance at some wavelength, .lambda.
.alpha.=extinction coefficient in m.sup.2 /g, PA1 C=smoke concentration in g/m.sup.3, and PA1 L=path length in m.
Other effective smoke-producing compositions are based on phosphorus compounds (particularly red phosphorus) which form phosphoric acid in the atmosphere. Typical red phosphorus (RP) smokes have an obscuration index of about 4000. Although phosphorus smokes are highly effective, the smoke products are extreme irritants and are corrosive. This has led the United States Surgeon General to require the use of gas masks by persons exposed to such smokes. In addition, phosphorus reactions typically produce intense heat which is a further hazard and limitation of this type of material.
There have been recent efforts to develop low toxicity smoke compositions based upon organic acids. For example, Douda et al. U.S. Pat. No. 4,032,374 discloses a low toxicity smoke composition based upon cinnamic acid for simulating fires and for training purposes. The cinnamic acid is volatilized by burning a mixture of potassium chlorate and sugar. Other low toxicity obscuring smokes based on aliphatic diacids are disclosed in Shaw et al. U.S. Pat. No. 5,154,782, which is incorporated herein by reference. In general, low toxicity smoke compositions based on organic acids have an obscuration index from about 120 to 140, approximately 60% of the screening power of HC smoke.
It will be appreciated that current low toxicity smokes are useful for training purposes, but not for battlefield deployment. This requires the military agency to maintain a training round and a field use round of smoke-producing compositions. It would be a significant advancement in the art to provide low toxicity smoke generating compositions that can be used for both training and field deployment. Reduced inventory costs and ability to train troops in the same smoke environment that would be encountered on the battlefield would be an important advantage.
Such low toxicity smoke generating compositions are disclosed and claimed herein.